1. Field of the Invention
This invention relates to an automatic equalizer which automatically equalizes transmission distortion or the like of a receive signal, and more particularly to an automatic equalizer suitable for use with a modem having a very high transmission rate,
2. Description of the Related Art
Various modems (modulator-demodulators) for use for transmission of data which can transmit data at various rates have been provided conventionally.
Generally, modems having a transmission speed of 9,600 bit/sec (9.6 kbps) or 14.4 kbps represented by the Recommendation V.29 have been put into practical use widely. However, as increase in transmission speed progresses in recent years, very high speed modems having a data transmission rate of 28.8 kbps are being developed energetically.
Modems as described above normally employ an automatic equalizer (which may be hereinafter referred to simply as equalizer) as shown in FIG. 11 in order to compensate, upon reception of data, for deterioration of the data (signal) caused by transmission distortion.
Referring to FIG. 11, the automatic equalizer is generally denoted at 102 and is interposed between an automatic gain control section (AGC) 101 and a signal discrimination section 103. The automatic gain control section 101 adjusts a loop gain so that the level of an inputted demodulated signal may be equal to a predetermined reference level to automatically adjust the level of a receive signal to be inputted to the automatic equalizer 102 at the succeeding stage. Thus, the automatic gain control section 101 is employed so that the automatic equalizer 102 at the succeeding stage may operate accurately.
The automatic equalizer 102 performs equalization processing to correct transmission distortion or the like of a channel. To this end, the automatic equalizer 102 includes an equalizer calculation section 1021 and a tap coefficient error correction section 1022.
Though not shown, the equalizer calculation section 1021 includes, as well known in the art, a plurality of delaying sections for delaying a demodulated signal from the automatic gain control section 101 in time, a tap coefficient multiplication section for multiplying delayed signals from the delaying sections by error information (an error signal) from the tap coefficient error correction section 1022 which will be hereinafter described, and a totaling calculation section for totaling results of the multiplications from the tap coefficient multiplication section. A result of the calculation by the totaling calculation section is outputted as an equalized signal.
The tap coefficient error correction section 1022 receives one of two branched outputs of the automatic gain control section 101 and error information (an error signal) from the signal discrimination section 103, which will be hereinafter described, and produces and outputs, based on the thus received signals, a control signal so that the values of the tap coefficients of the equalizer calculation section 1021 may be corrected to optimum values.
The signal discrimination section 103 performs pre-processing for signal discrimination at a succeeding stage and outputs false reference signal corresponding to an input signal thereto. The signal discrimination section 103 also produces and outputs a false reference signal for the tap coefficient error correction section 1022 of the automatic equalizer 102. Further, the output of the automatic equalizer 102 and the discrimination result output of the signal discrimination section 103 are negatively added to each other (subtracted from each other) to obtain an error signal, which is outputted to the tap coefficient error correction section 1022. An adder 104 is connected so as to negatively add (subtract) an output signal of the signal discrimination section 103 to (from) an input signal to the signal discrimination section 103 to obtain an error signal to be supplied to the tap coefficient error correction section 1022.
Due to the construction described above, with the automatic equalizer shown in FIG. 11, a demodulated signal whose gain is kept fixed by the automatic gain control section 101 is input to the equalizer calculation section 1021. The equalizer calculation section 1021 corrects tap coefficients to optimum values in accordance with a control signal obtained from the tap coefficient error correction section 1022 to per form accurate equalization of the input signal.
In this instance, in the tap coefficient error correction section 1022, required processing such as multiplication or addition is performed on a signal before equalization to be input to the automatic equalizer 102 and an error signal obtained by negative addition of the output of the automatic equalizer 102 in the past and the discrimination result output of the signal discrimination section 103 by the adder 104. A signal obtained by the processing is outputted as a control signal to the equalizer calculation section 1021.
In short, in the conventional automatic equalizer, input data (an input signal) to the automatic equalizer 102 and input data to the tap coefficient error correction section 1022 which is used for error correction are used commonly.
In order to use the automatic equalizer 102 of such a construction as described above for a very high speed modem having a transmission rate of, for example, 28.8 kbps, it is necessary to increase the number of taps of the equalizer 102 very much and lower the level of input data so that the loop gain of the equalizer 102 may be kept equal to or lower than 1.
However, if the level of input data to the automatic equalizer 102 is lowered in this manner, then the accuracy in error correction of the tap coefficients by the tap coefficient error correction section 1022, in which the input data is used, commonly is degraded, and also the accuracy of the automatic equalizer 102 is degraded, which is a subject to be solved with the conventional automatic equalizer.